Constant envelope signals are a popular way for transmitting wireless or over-the-air radio frequency (RF) signals. For a constant envelope signal, the envelope of the carrier wave does not change in response to changes in the modulated signal. In other words, the maximum and minimum amplitude of a constant envelope signal is kept at a constant level. Constant envelope signaling schemes are advantageous in that they are efficient from a transmitted power standpoint. This is because constant envelope signals allow a transmitter's power amplifiers to operate at or near saturation levels, which correspond to the point whereby the power amplifiers operate at peak efficiency. Furthermore, due to the fact that the amplitude is maintained at a constant level, the power amplifiers only have to provide a steady amount of amplification. Consequently, there are less non-linearities and signal distortions associated with the amplification of constant envelope signals.
In contrast, variable envelope signals have envelopes that change over time. Variable envelope signals can transmit a greater amount of data for the same occupied frequency bandwidth over a given amount of time as compared to constant envelope signals. This results in improved spectral efficiency. Unfortunately, power amplifiers for amplifying variable envelope signals operate at an average power level which is significantly less than their peak power. This means that the power amplifiers are mostly operating at a point which is less than ideal. This reduces the power efficiency of these variable envelope power amplifiers. Furthermore, power amplifiers for variable envelope signals change the signals' amplitudes by varying amounts, depending on the instantaneous amplitudes of the signals. The greater the degree that the amplitude of a signal varies, the more non-linear amplification is exhibited. This non-linear amplification produces distortions in the variable envelope signal and non-idealities in the channel. Such distortions and non-idealities could cause errors in the receiver. The received data could become corrupted, and the transmitted distorted signal will experience spectral regrowth.
Thus, wireless communications designers face a dilemma. The designers can implement constant envelope signals, which are highly efficient from a power standpoint and are also less susceptible to distortions. However, the trade-off is that constant envelope signals cannot transmit data as fast as compared to variable envelope signals. Although variable envelope signals have better spectral efficiency, this comes at the expense of reduced power efficiency and increased susceptibility to signal distortions and non-idealities which could ultimately lead to receiver errors and unacceptable out-of-band spectral emissions.